Telephone transmission system



Jan. 11, 193 4 HH MITH 2,105,016

TELEPHONE TRANSMISSON SYSTEM Filed Jan. 2, 1936 4'Sheets-Sheet l INVENTOR. HOWARD H. SMITH ATTORNEY.

Jan. 11, 1938. H. H. SMITH 2,105,016

- TELEPHONE TRANSMISSON SYSTEM Filed Jan. 2, 1936 4 Sheets-Sheet 2 INVENTOR. HOWARD H. SMITH ATTORNEY.

Jan. 11, 1938. H, H, SMITH 2,105,016

TELEPHONE TRANSMISSON SYSTEM Filed Jaxi. 2, 1936 4 Sheets-Sheet} J! I LEI I EXCHANGE I INVENI OR. HOWARD H. SMITH ATTORNEY.

Jan. 11, 1938. s rr 2,105,016

TELEPHONE TRANSMI S 8 ON SYSTEM Filed Jan. 2, 1956 4 Sheets-Sheet 4 TRANSMIT EXCHANGE 2 INVENTOR HOWARD H. SMITH ATTORNEY.

Patented Jan. 11, 1938 Howard'H. Smith, Chicago, 11]., 388131101 to Associated Electric laboratories, Inor, Chicago,

11L, a corporation of Delaware Application 2, 1936, Serial No. 57,116

The present invention relates to telephone transmission systems, but is concerned more particularly with multi channel telephone transmission systems. .i

5 The object of the invention is. theproduction 7 of a new and improved systemfor providing for I mulgi-channel telephone transmissionf GENERAL DESCRIPTION Thesystem disclosed herein, like previous systems} for providing multi-channel operation over the Some common circuit path, isbased onthe fact that -a,telephone'-ci rcuit may serve as a; transmission medium for'a frequency band many times wider than the frequencyband-necessary for ordinary telephone conversation. It has been found that if a band-of voice frequencies 3000 about 3200*cycles is transmitteduniformly to the exclusion of another frequencies which may be contained in the voice of a speaker, a commercial quality of intelligibility results. Now, an inter-exchange or inter-city telephone circuit is ordinarily capable of transmitting or carrying ffrequencies satisfactorily up to 30,000,

cycles or higher. It is obvious, therefore, that,-

a suitable transmission system can be devised, I several conversational channels may be secured by arranging that the extended frequency-transmitting characteristics of the telephone circuit be utilized.

. The novel system disclosed herein is based on the principle of multiplying all of the necessary frequencies of a telephone conversation by a suificiently la'rge common factor that theresulting transmission frequencies compose a band of a l width commensurate'with the frequency-carrying' capacity of the circuit. This common factor may be five, for example, if the chosen upper frequency limit of the circuit is slightly in excess of 16,000 cycles, and if 3200 cycles is. chosen as the upper limit of voiceinput. 'I'hisfr'equency multiplication is obtained by first recording a small portion of a-train of voice currents; reproducing these recorded voice currents at a rate which is five times as' fast as they'wererecorded; and sending the reproduced high-frequency cur- -rentsovertheline.' l

1'IFhereeording may take place continuously, so while thereproducing and transmission over the line/ takes place intermittently, onefifth the line time. Allowing-for inter-channel switching time intervals, it has been found that underthe assumption above made four converj sations' nay be sent over the line during the.

. vi Claims. (01. me ls) sub-intervals of receiving time.

'tributor DI, Fig. 2 is a side view of the receiving same time period by arranging thatthe times of reproduction and retransmission of the high frequency currents resulting from the four conversational records be interspersed. At the distant end of the circuit, the incoming higlr fre- 5 quency voice currents are recorded intermittently as concerns any individual conversation, and are reproduced at the original recording rate, wherev by h reproduction of the received and recorded currents'is continuous, and at the original rate. 10 -By making the time units involved sufllciently short, the 'delay occasioned by the recording and reproduction may be reduced suflicie'ntly that the persons conversing are not aware that the transmission is not proceeding uninterruptedly. I 15 With the novel-arrangement just outlined, the

time elapsing during 'the transmission of tele cycleswide, extending from about 200 cycles to 0 phone conversations is divided into small unit intervals, and each unit interval is further subdivided into four sub-intervals, each sub-interval being. allotted to one of the conversing subscribers. This allot'ting, together with the recording and transmitting over the commonline is accomplished by what may be termed a distributor.

This distributor allots regularly recurring'intervals of time to each of the four incoming channels, while a receiving distributor at the distant end of the line makes a similar allotment of the It has been chosen to illustrate the invention a as applied in obtaining four-channel, two-way telephone transmission over an inter-city telephoneline. Y Y a r A Description of drawings lftefe'rrin'g now to the accompanying drawings, Fig. 1 showsan end view of the mechanical construction of a combined transmitting and receiving distributor designated generally as the disdistributor DI, Fig. 3 is a circuit drawing showing the distributor DI used asthe transmitting and receiving distributor atone end of the telephone line TL, at the other end of which the-dlstributor D2 shown in Fig. 4 operates to receive from the transmitting portion of the distributor DI and to transmit to the receiving portion of the distributor m. a v v r Dm'ArLm Drscalr'non The invention havingbeen described generally, a detailed description will be given, for which purpose the mechanical construction of distributor DI will flrstbe described 7 The distributor D1 'hollowshaft 3, which in turn is journaled to rotate within the sleeve I3,supported by the housing I2. The shafts 2 and 3 are arranged to be rotated in opposite directions and at different speeds by the motor 4, on whose shaft 5 the gears 6 and 8 are mounted. The gear 6 drives a reducing gear 'i-which is rigidly secured to the central shaft The motor 4 'is' assumed to be rotatable at a constant speed in a clockwise direction as viewed from the left, whereby the central shaft 2 rotates in a counterclockwise direction.

The gear a, rotating in a clockwise direction because it is secured to shaft 5 of the motor, drives the reverse gear 9 in a counter-clockwise direction, whereby the auxiliary gear Ill is driven in the same direction. The gear I0 drives the reducing gear II in a. clockwise direction. Since the gear I I is secured rigidly to the hollow shaft 3, the shaft 3 is rotated in a clockwise direction, but at a much lower rate than the rotation of the central shaft 2 ma counter-clockwise direction.

The track wlieel I4 is secured to the hollow shaft 3, at the left end of the shaft, and the brush wheel I5 issecured to the left end of the central shaft 2, whereby the wheels I4 and I5 are rotated respectively by the shafts 3 and 2, the wheel I 4 revolving relatively slowly in a clockwise direction while the brush wheel I5 revolves at a relatively rapid rate in a counter-clockwise direction.

The track wheel I4 has the track disc I8 clamped between its rim and the disc clamp I9 to provide two sound tracks on the disc I18," an

outer track for temporarily recording telephone conversations to be transmitted over a. multichannel line, and an inner track for temporarily recording conversations received over the multichannel line before they are placed on local single-channel lines. The two sound tracks on the disc I8, carried by the track wheel I4, are separated by the stiffening rings 20 and 2I which are provided to render the relatively thin track disc l8 sufliciently still that it is not distorted materially by the pull of the erasing electromagnets which arehereinafter described. The track disc is made thin enough that a sound record magnetically impressed on one side may be reproduced by passing a pickup coil along the other side.

As the disc I8 is revolved in a counter-clockwise direction, and considering now the outer or transmitting sound track (lying outside the stiifening rings 20 and 2|) the disc at any instant may be considered as 'divided into quadrants by the four groups of electromagnets mounted on,

the stationary sending ring 5.. Each of these groups of magnets corresponds to a, separate, tele-' phone transmitting channel "as is indicated in.

7 Figs. 3 and 4, wherein the channels have been designated as A, B, C, and D, respectively. Magnet 4.I is one of the three horseshoe electromagnets of channel A; magnet 42 is one ofthe three magnets of channel B; magnet 43 is one of the three magnets'of channel C; and magnet is one of the three magnets of channelD. Voice currents passing through the coil '4I asthe disc I8 is rotated in a clockwise-direction leave amag netic voice record on the outer sound track of disc I8, as the disc- I8 is assumed to be made of magnetic material of relatively highretentivity;

, tionary contact brushes 26. mounted" so as to counter-balance the electroon the disc, which impression or record is ex' tended until the portion in question starts to approach the group of magnets containing magnet 4I. When magnet 34 of this group is reached,

the record is completely removed from the sound track because the portion of the track under the magnet 3t is saturated by the strong magnetic flux emanating from the poles of magnet 34. As the portion of the sound track travels still further, magnet 35, which is less strongly magnetized than magnet 34, and in the reverse direction, removes the effect of the saturation of the sound track and restores the portion to a normal non-magnetized state. Therefore, when the portion of the disc in question progresses'further and passes underthe recording magnet M of channeLA, all evidence of the previous sound record has been removed and the portion of the tinuous -motion and, is therefore continuously moving the track disc I8. As a result, the portion of the disc between a recording magnet, such as 42, and the next succeeding erasing magnet,

such as 34', contains a. portion of a. spoken mesf.

sage the length of which depends upon the amount of time required for the disc 'to travel from the recording magnet to the next succeeding erasing magnet. Therefore, in order that none of the recorded speech be1lost, it is essential that a. reproduction be made of what is recorded once during each successive time interval of a length no longer than the length of time required for a portionof the discto travel from a recording magnet such as 42 until it comes under the influence of the next succeeding erasing magnet such as 34.

The reproduction and transmission of th recorded voice currents is accomplished by the transmitting reproducing magnet'28, which is mounted on the .rim of the brush wheel I5 near its periphery and counterbalanced bythe weight 30, mounted at a point diametrically opposite the horseshoe electromagnet 28. The reproducing, transmitting, magnetv 28 is maintained inconstant connection with the transmitting circuit 'through the medium of the slip rings 25 and the stationary contact brushes 2!, while the 'r'eceiving, recording magnet 29, mounted near the.

inner edge "of therim' of the brush wheel I5 is similarly maintained in connection with the receivingjc ircuit through the rings 21 and the sta- The weight 3| is ma n tv Z9.

.Since the mitting distributor and receiving distributor, it is providedfwith receiving, reproducing stationary magnets in addition'to. the transmitting, recording stationarymagnets. These receiving,'recording magnets for channels A, B, C, and D are respectively II, I2, 13, and 1-4, all of which are shown in Figs. 3 and 4, in-addition to being shown in Figsil and 2. These magnets are mounted on the adjustable stationary receiving ring I'I, whose distributor DI is a combined trans- ,adjustable feature is provided for a purpose which will be made clear in a subsequent part of the description. Adjustment of the receiving ring I! is provided by'turning the worm 22 in the desired direction, such as by inserting a screw driver or similar instrument lathe-slotted end of the shaft which supports the worm 22. The worm 22 cooperates with the worm'gear cut on the periphery of receiving ring II, which ring is rotatably mounted on a. portion ofthe bearing ,sleeve i3. The group of magnets carried by the one of the jacks at the manual switchboard of.

exchange 2.

receiving ring" may thereby be adjusted with respect to the groups of magnets carried by the transmitting ring l'6. T

A further feature of sociated worm gear secured around the housing of the driving motor 4. The motor 4 is mounted in brackets 32 and 33 so that it can be rotated bodily when the worm 23 is turned in the desired direction, which turning is facilitated by the slot in the end of the supporting shaft. The desirability of this adjustment will be made apparent hereinafter. e

' The system Referring now particularly to Figs. 3 and 4, it is to be noted that the telephone line TL extends between exchange 1 and exchange 2. These exchanges are assumed to be' located in diflerent cities, whereby the interconnecting lin'e TL is suificiently valuable because of its length that it .is economical to supply additional terminal ,equipment in order to provide the additional telephone channels. A manual switchboard is assumed to be provided in each of the exchanges. The jack J l is one of the jacks at the manual switchboard of exchange 1, while thejack J2 is These two Jack's, as will be more fully'pointed out hereinafter serve as the termimale of one of the channels (channel A) which is providedover'the common line TL; The line equipment LEI is associated with the lack Jl.

- Thisline equipment may be any standard equipment such as is used with carrier-current telephone systems, and it is arranged to close a lighting circuit for the lamp Ll when the operator at exchange 4 is to be signalled from exchange 2,

- which signalling is carried on by signalling cm- 60 line equipment LEI and the distributor DI. This rent in the range. of voice frequencies, as is well known. The line equipment LE2 and the lamp L2 are associated with the. jack J2 in the second exchange to perform similar functions. Each of the three other two-.way channels provided over the trunk' line TL may be. similarly equipped, although the jacks and associated line equipment of these'other channels have been omitted for the sake of simplicity.

The balance coil 55 is introduced between the balance coil, along with theartificial line 56 operates in the well-known way to provide separate voice channels outgoingirom and incoming to I magnet TI to the lower winding of coil 55. When 7o received from the reproducing magnet 'll are 're-' peated-alike to the artificial line "and the local .76

the jack Jl, The outgoing channel extends from the mid-points of the line windings oi'the balance-coiljto the recording magnet 4|, while the incoming channel extends fron the reproducing the artificial. line, 56- is properly balanced to match the local line connected with the jack J I byway of the manual switchboard. voice currents line, and novoice currents are driven through g adjustment of the distributor is, provided by the worm 23 and the as- 3 the transmitting branch to,the recording magnet 4|.

wise voice currents received from the reproducing magnet ll would be transmitted to the recording magnet 4| and sent back over the circuit in the opposite direction, which would tend to produce undesirable singing. The balance coil 65 and the artificial line 66 are provided in association with the jack J2 for the same purpose.

At the distributor DI, it is to be observed that the windings of the erasing electromagnets, such the disc then come under the influence of the restoring magnets such as 35. These restoring magnetsare connected in series with .each other and are supplied with current from the exchange battery byway of the regulating rheostat 45. As is evident from the fact that the connections are madein reverse direction to the restoring magnets, the magnetic flux emanating from the restoring magnets is in the opposite 'direction to that emanating from the saturating erasing The reason for this arrangement be-' comes apparent when it is pointed out that othermagnets. The result of this is thatthe portionofthe disc passing under a magnet such-as would be demagnetized and then saturated in the opposite direction if the restoring magnets were of the same strength as the erasing magnets. However, the restoring magnets are of lesser strength than the. erasing magnets, for which reason the previously efiected saturation is undone only to the extent that the portion of the sound track coming from under the influence of a restoring magnet such as 35 has little or no magnetism. This condition may be regulated somewhat by an adjustment of the rheostat 46. a

Considering the receiving portion-of the distributor DI, the erasing magnets'such as 36 are connected in a series circuit controlled by the rheostat 41, while the restoring magnets such as 31 are connected -in a' series circuit controlled by the rheostat 48.

As previously pointed out, the magnet 28 is the travelling reproducing transmitting magnet, be-

' ing mounted on the brush wheel |5,.Fig. 1, so that it moves relatively rapidly in a counter-clock-- wise direction, while the magnet 29 is the travelling receiving recording magnet, also'moving in a counter-clockwise direction. The amplifier 51 is the outgoing line amplifier used to amplify the high-frequency outgoing voice .currents before line AL is provided tobalancethe characteristics of the trunk line TL, whergbythe balance coil ,5. may serve to prevent outgoing voice currents'i'rom reaching the-incoming branch associated with-the ampliflerjfl, as discussed in connection with the balance coil 55. The incoming voice currents; on the other hand, are transmittedthrough the incoming line amplifier ,which leads into the common receiving recording magthey reach the common line TL. The artificial net 23 by way. of brushes ,2 and slip-ring 21. In

'exchange'2, theline TL is provided with balance coil ll, artificial lineill, input amplifier "I, and

output amplifier 19, corresponding respectively,

to the parts 58, 59, 51, and 60 in exchange 1.

The distributor D2 may be like the distributor 'DI; the transmitting and receiving magnets 38,

magnets 4| to 44 of the distributor Di; and the stationary reproducing magnets 5| to 54 associated with the receiving portion of the distributor D2, correspond respectively to the stationary reproducing magnets H to 14 of the receiving portion of the distributor DI.

Operation of the system For the successful and satisfactory operation of the system; it is. necessary that the synchronous motor 4 of the distributor DI drive its shaft 5 in synchronism with the corresponding shaft oflthe distributor D2. This result may be obtained, for example, by driving the motors from the same system of alternating-current power distribution, or the driving system may depend largely upon local powerwith a speed correction obtained over an interconnecting line as is done for instance'in multiplex telegraphy or in systems for maintaining synchronism in the transmission of still pictures by wire. When the system is tobe started into operation, the motors of the distributors at the two exchanges; are started and are caused to run in synchronism.

Phasing the distributors In order that the system operate as intended, it is further necessary to bring each receiving distributor into phase with the transmitting dis tributor from which it receives. For this purpose,

a source of tone current may be connected to,

the tip and ring conductor of the jack J i, while the person in charge of the equipment at exchange 2 connects a receiver to the jack J2 and listens for the tone which is appliedthrough the -jack J I. The receiving ring of the distributor D2 (corresponding to ring I! of the distributor DI) is then adjusted by the worm and gear arjustment may be verified by associating the test receiver with the other three jacks (not shown) corresponding to channels B, C, and D. When the adjustment is correct, none of the 'tone applied to jack Jl can be heard at any other one of the jacks. I

Thereceiving portion of the distributor Di may be brought into phase with the transmitting portion of the distributor D2 by a similar method. For this purpose, the tone source may be connected to the talking conductorsat the jack J2, and areceiver may be connected to the jack J i The worm 22 is then turned in one direction or the other until the worm gear cut on the'periphery of the-receiving ring I'I drives the ring into the correct position,- in which\position the tone applied at the jack J2 will be heard at the jack J I as a continuous tone and will not be heard at all at any one of the other three jacks associated with the system and corresponding respectively to channels B, C, and D. When this has been accomplished, the four two-way channels over the inter-exchange trunk line TL are available for service.

Setting up connections As is indicated by the commoneconnection symbols associated with the jack J i, this jack may be I multipled with other positions of the switchboard so that the associated channel may be used by any one of the number of operators. Assuming that a.

call is received at exchange 1 and is to be completed through exchange 2, the operator at exchange 1"who receives the call may makethe usual tip busy test on the sleeves of the jacks such as J]. Assuming that the jack J i tests idle, the

operator may insert the plug (notv shown) of hercordcircuit into the jack J i, causing this channel thereby to. test busy at the other test positions. The operator may now project ringing current over the channel A through the jack J i I It is'assumed that the line equipment LEi contains equipment similar to the so-called composite ringing equipment for responding toringing current by relaying current at a selected frequency within the voice range for signalling purposes This relayed current traverses the channel-.A portion of the distributor Di and passes over the trunk line TL and is repeated through the chan- 'nel-A- receiving portion of the distributor Di to the line equipment LE2 associated with the'jack J2. Theline equipment LE2 is assumed to have responding-equipment therein which responds to the relayed ringing signal by lighting the line lamp L2 as a .call signal to the operator. The operator in exchange 2 may now respond to thissig nal by inserting the plug of an idle cord circuit into the jackJZ to extend the connection.

Ring-01f and other supervision may be controlled by relayed signal currents lying within the voice-frequency range, as pointed out in connection with the original signaling operation above described. I 7

Transmission of voice currents nels, A, B, C,'and D, of the line TL, under which condition the maximum use may be made ofthe trunk line TL. A description will now begiven of theoperations that take place during one or more cycles of operation of the distributor. It

may be assumed that each of the four subscriber's in exchange 1 is talking at the moment thedescription covers. Under this condition, voicecurrents are being received continuously at each of the recording magnets M, 42, 43, and 44 of the transmitting portion of the distributor Di the outer soundtrack of the track disc i8 is being driven continuously in a clockwise direction under the pole pieces of the magnets II to M at a rate which is suflicient to elongate the recorded magnetization curves sufliciently. that they may be reproduced clearly. At the same time, the transmitting reproducing magnet 28 of the distributor DI .is being driven in a counter-clockwise direction at an angular velocity' about five times that of the track l8, whereby the magnet 28 makes one complete revolution in the timerequireddor aportion of the disc l8 to pass from under the poles of a recording magnetsuch as 42 and travel 4 far enough to reach the influence of the next succeeding erasing magnet such as 34. Considering now channel B in particular, and at a time when.

the reproducing transmitting magnet 28 is passing over the pole pieces of the recording magnet 5 42, the portion of the outer sound track of the disc l8. which is'at that moment under the pole pieces of magnet" travels until it is about to come under the influence of the erasing magnet 34. At this point, themagnet 28, which in the.

meantime has rotated around over channels C,

frequency interspersed message groups are sorted net 28 proceeds a little further and has left the D, and A, comes from the field of influence of the magnet 34 in time to resume the reproduction and transmission of-the record produced by magnet 42 before an unreproducedportion of the message .is erased. The next portion, therefore, of the message being received over channel B from the local subscriber in exchange 1 is quickly transmitted over the line by the reproducing and transer or preferably by enlarging the radius of the.

brush wheel 28 and of the magnet rings l8 and I1, along with the radius of the track disc l8. It is to be observed further that, although a considerable disturbance may be created when the traveling magnet 28 passes over an erasing magnet such as 35, the current resulting from this disturbance reaches the distant receiving magnet 39 at a time when this magnet also is passing over an erasing magnet, for which reason the disturbance is not recorded at the distant end and does not therefore appear 'in the conversation heard by the distant subscriber in association with whose channel the disturbance may be created.

Reception of voice currents Considering now the way in which the highout and reassembled into separate continuous ,messages at the distant exchange, it will berecalled that the distant receiving recording magnet 39 'is'being rotated in 'synchronism with the reproducing transmitting magnet 28 and in phase with it except for the small time-interval dis placement representing the transmission time of the voice currents over the line TL from exchange 1 to exchange2. Consider now the sub-interval of time which begins when the transmitting reproducing magnet 28 of the distributor DI is leaving the influence of erasing magnet 86 'in the channel-B group. When the surge of current tran'smitted'as a result of magnet28 passing over this erasing magnet reaches the distant exchange,

magnet 39 is passing over the erasing magnet 8| in the channelec group of magnets. When maginfluence of the above-mentioned erasing mag- .net, magnet 39 is passing over the pole pieces of the-stationary reproducing: magnet 53 of receiving channel C in the distant exchange. Then, as magnet 28 progresses further toward the channel C magnet 43 it transmits the record of the voice currents that have been received over the channel C circuit in exchange 1 duringthe time interval which haselapsed since magnet 28 previously contacted the channel-C portion -of the record. At

thesame time receiving recording magnet 39 of the distributor D2 isproceeding'tqward the channel D magnet group. including magnet SI. Just asrnagnet 28 arrives 'over the pole pieces of the channel D stationary magnet 43, magnet 390! .the distributor D2'comes under the influence of the restoring. ma net in; the group containing magnet 54. Then, while tlieportionofthe track on which a recordihas impressed-by the receiving recording magnet 29 during its travel in the C quadrant is reproduced and sent over the C channel to the subscriber in exchange 2, magnet 39 travels over the other three quadrants. It comes again into effective relationship to quadrant 0 just as the last of the previously recorded portion of the message is reproduced. In this way, a continuous reproduction and transmission to channel C branch in exchange 2 isaccomplished. A similar result is accomplished in the same way for each 'of the other three channels.

Two-way operation While the transmission over the four channels from exchange I to exchange 2 has been taking place as'above described, the transmitting portion of distributor D2 has been cooperating with the receiving portion of the distributor DI to effect a similar four-channel transmission in the reverse direction in the event that any voice currents are generated on the respective channels in exchange 2. It will be understood of course that, if the subscribers in exchange 1 cease talking and the subscribers in exchange 2 all start talking, the four-channel operation proceeds in the-reverse direction in the manner described. However, it makes no difierence'to the operation of the distributor system whether ornot voice currents are travelling in the same direction over all the channels or even whether or not they are travelling in only one direction over one channel at a time or not. Voice currents may be transmitted simul taneously in both directions over each of the channels A to D if all of the subscribers should be talking at once, which discussion is largely of academic interest except as it concerns the operation which takes place when a listening subscriber desires to interrupt the conversation of the other, possibly to interject a question. Inshort, the subscribers interconnected by way "of,

, sage to be sent through the transmitter and recorded and repeated at the receiving distributor.

Stopping and restarting In thev event that the system'is to be stopped, which may logically occur during a period of light traflic, the power supply of the driving motors maybe disconnected. When the system is to be.

restarted the two distributors may be brought into phase by rotating the frame structure of either of the driving moto'rs.' For example, by turning the worm 23 slowly the frame of the motor I is roltated in the proper direction to bringthe centralshaf t' 2 andthe hollow shaift'3 into the same phase relationship with respect to the corr sponding shaftso'n distributor D2 as existed during the previous operation; When, the two distributors have been brought, into the same phase relation that previously existed, the receiving and transmitting-portions of .allthe channels in birth directions will b in phase, 'andfindividual adjustments at the tw'o'di'stributors may be done away with. This operation will be understood when it is recalled that a synchronous motor the speed ofits rotating shaft constant with respect to'the frame structure of 'the'motor; Therefore, if the frame-structure of the motor is turned so slowly asnot'to impose an'overload on-the motor,

"the speed 0! the motor shaft is increased or deshaft running ata constant speed with respect to the frame of the motor, as the-motor frame is turned in one direction or the other in order to bring the rotating parts of the distributor into the previously existing phase relationship.

The separate adjustments of the distributor may be made use of also in order to bring the dis- 1 tributors into the proper phase relationship with the magnet groups of the receiving ring of each distributor sufficiently displaced with respect to the magnet of the associated transmitting ring so as to minimize any tendency for direct inter group aerial magnetic inteference.

Modified arrangements a The system herein disclosed may obviously be refined in certain respects. For example, the amplifiers 51 and 61 may be made to include a band pass filter which will eliminate from the reproduced conversation all frequencies which exceed a set maximum, whereby the main'mum' frequency transmitted over the line 'I-L from the distributor DI is limited. This arrangement may be employed to prevent excessive cross-talk interaction between adiacent lines similarly equipped,

in which case the amplifier Bl issimilarly-prmvided with an inputband pass filter to limitthe frequency of the currents impressed on the line TL at exchange 2.

A modification which at once suggests itself may include the use of a four-wire circuit between the two distributors to replace the line TL, transmission in on'e'direction proceeding over one pair of wires and transmission in the other direction proceeding overthe other pair. of wires. This arrangement may be used to advantage if the line TL is sufficiently long that a great number of repeaters are required, in which case separate balancing networks at the various repeaters are done away with and simple one-way ampli fiers may be used, as is well known.

It will be understood that the switchboards in the two exchangesmay be automatic switchboards instead of manual switchboards, in which case dial impulses may be transmitted over the channels by any known method of voice-frequency' dialling, such as. the system disclosed ing track thereon, means -for magnetically recording more than one conversation on the transmitting track of each disc, means for transmitting all the recorded conversations over said trunk line at a higher; speed than that at which they were recorded, means for recording a plurality of received conversations at the same time on the receiving track of each disc as received over said trunk line, and means for reproducing the conversations recordedon the receiving track, whereby a plurality of two-way conversations may be carried on over said trunk line without interference. I I

=2. In a multiplex telephone transmission system, a two-conductor trunk line, a distributor at each end thereof, a plurality of telephone stations connected with each distributor, each distributor including a rotating disc having a transmitting and a receiving track thereon, each station having a magnet for recording outgoing voice currents on the transmitting track of'its distributor, and a magnet for reproducing recorded voice currents from the receiving track, another element rotating at a greater speed than said disc, said element carrying a transmitting device which transmits outgoing voice currents recorded on the disc by the magnets of all stations, and a receiving device which records incoming voice currents on the receiving track of the disc for reproduction by all the reproducing magnets. I

3. In a multiplex telephone system, a plurality of telephone lines, a trunk line, a distributor for recording and retransmitting incoming and outgoing conversation from the lines over the trunk line and from the trunk line to all the lines si multaneously, said distributor-comprising a rotating disc adapted to receive and retain magnetic impressions of voice currents received from and transmitted to the trunk line, a magnet for each line for-recording on the disc and a magnet for each line-for reproducing from the disc, said magnets regularly spaced around the periphery of the disc, and a pair of magnets for the trunk rotated relative to the disc for reproducing re periphery on which voice currents from all the lines are simultaneously recorded and reproduced and transmitted over the trunk line, said disc having another sound track, concentric with the first, and means for recording all voice currents received from'the trunk line, on said last soundtrack and reproducing and distributing them to the various lines.

HOWARD H. SMITH. 

